Process for making hyperpure gallium

ABSTRACT

A method for preparing hyperpure metal selected from the group consisting of gallium, germanium and indium comprising the steps of: halogenating one of said metals which contains less than about 2 percent impurities; dissolving the halogenated metal in liquid ammonia; filtering undissolved solids from the solution; evaporating the solution to dryness to provide solid material; redissolving the solid material in liquid ammonia; electrolyzing the resolution of the solid material; and, recovering hyperpure metal from the cathode of the electrolytic cell.

D United States Patent 1 1 1 3,897,317

Bawa July 29, 1975 [5 PROCESS FOR MAKING HYPERPURE 3,325,380 6/1967 Leibenzeder 204/59 M 3,325,383 6/1967 lwantscheff et al 204/105 R [75] Inventor: Mohendra S. Bawa, Plano, Tex. Primary Examiner F' C. Edmundson [73] Assignee: Texas Instruments Incorporated, Attorney, Agent, or Firml-larold Levine; James T.

Dallas, Tex. Comfort; Gary C. Honeycutt [22] Filed: June 24, 1974 [211 App]. No.: 482,032 [57] AFSTRACT A method for preparmg hyperpure metal selected from the group consisting of gallium, germanium and [52] US. Cl. 204/59 M; 204/105 R indium comprising the Steps halogenating one of C22) 9/00; C22b 41/00; C22b 61/06 said metals which contains less than about 2 percent [58] Field of Search 204/59 59 105 impurities; dissolving the halogenated metal in liquid ammonia; filtering undissolved solids from the solu- [56] References Cited tion; evaporating the solution to dryness to provide UNITED STATES PATENTS solid material; redissolving the solid material in liquid 2,313,408 3/1943 Vingee ct al. 204/59 AM ammonia; electrolyzing the resolution of the Solid 2,615,838 10/1952 Minnick ct al.. 204/59 AM terial; and, recovering hyperpure metal from the cath- 3,075,901 1/1963 l-lutter et a1 204/105 R ode of the electrolytic cell. 3,167,422 1/1965 lwantscheff et al. 204/59 M 3,170,856 2/1965 Leibenzeder 204/59 M 3 Claims, N0 Drawings 1 s PROCESS FOR MAKING HYPERPURE ,GALLIUM DESCRIPTION OF THE INVENTION This invention relates to a process for the production of hyperpure metals useful in the electronic andsemiconductor arts. More particularly, the invention relates to a process for the preparation of hyperpure gallium by selective solubilization of gallium compounds in liquid ammonia followed by electrolytic reduction of the pure gallium compound. Gallium of this purity is also referred to as electronic grade gallium and is useful as a component of compound semiconductors.

In the art of semiconductors, the active element, such as gallium, is generally required to have a very low impurity content,,namely in the order of about to 10 (100 .001 ppm) parts by weight for every part of gallium used in the compound used in the semiconductor. The herein disclosed process provides a method for preparing metallic gallium having an impurity content in the range stated.

Conventional processes are used to refine or reclaim metallic gallium. These processes include electrolytic reduction of such gallium compounds as sodium gallate, i.e., NaGaO or Na GaO and are well known in the art. Metallic gallium, subsequent to being refined or reclaimed by means of conventional'processes, generally has a purity content of between about 99 percent and 99.9 percent by weight. The purity may be as low as 98 percent. The remaining .1 percent 2 percent is made up of residual metal and other impurities. Since this level of purity is inadequate for semiconductor work, further methods of purification have been devised to reduce the impurity content to within the range stated. Reference is made to the text Pure Chemical Elements for Semiconductors by Marshall Sitig, 1969, Noyes Development Corporation, Parkridge, New Jersey, and particularly to pages 39-70 thereof which is a compilation of methods described in certain United States patents for purifying gallium. For example, U.S. Pat. No. 3,075,901 discloses a process wherein a halogen gas is bubbled through molten gallium to provide a gallium-halide slag on the surface of the molten gallium. The reaction products are gallium-halide salts which are distilled from the molten mass and condensed and dissolved in an aqueous solution in purified form. U.S. Pat. No. 3,170,856 describes an electrolytic process wherein a galliumhalide complex is dissolved in an organic solvent from which gallium metal is reduced in the cell described therein. Other electrolytic methods are disclosed in U.S. Pat. Nos. 3,325,830; 3,423,301; and 3,192,139. Other processes in the prior art include disproportionation of gallium chloride using purified hydrogen gas (Pat. No. 3,150,965); reacting impure gallium with hydrochloric acid to form gallium GaCl; and thereafter reacting that gallium with purified gallium metal to form GaCl which is disproportionated at high temperatures into purified gallium metal and gallium trichloride (U.S. Pat. No. 2,928,731). Other methods include decomposition of gallium alkyls (U.S. Pat. No. 2,898,278); and recrystallization purification (U.S. Pat. No. 3,088,853).

The present invention provides a process for preparing hyperpure gallium metal by means of the new and novel process herein disclosed. The process of this invention comprises the steps of reacting commercially pure gallium metal to form gallium trihalide (GaCl and thereafter dissolving the gallium trihalide in dry,

pure, liquid ammonia. The gallium chloride/ammoniate solution is then filtered to remove undissolved solids and the solution is thereafter evaporated to dryness. The ammonia dissolution/filtrationlevaporation steps may be repeated several times depending on the quality of purity required. It is preferred that the ammonia solution/filtrationlevaporation steps be continued until no further solids remain undissolved in the gallium chloride/ammonia-te solution. After the last of the required evaporation steps, fresh, purified and dried liquid ammonia is used to redissolve the solids from the last evaporation step of the ammonia solution.

The gallium chloride/ammoniate solution is then electrolyzedin a suitable (inert) container wherein the gallium compound is reduced to pure gallium metal at the cathode. The purity of the gallium metal produced by this process is in the range of the hyperpure gallium previously described.

Gallium sludge may contain up to 1 percent and greater of impurities such as lead, zinc, calcium, aluminum, boron, copper, vanadium and other metals. Preferably the crude gallium sludge which contains such impurities is heated to form a melt and is then treated with a halogenating agent such as chlorine gas, methyl chloride, hydrogen chloride or other halogenated hydrocarbon which are useful as halogenating agents. It is believed that the haloganation reaction proceeds to form gallium trichloride. In place of the chlorine, the corresponding bromine, iodine and fluorine compounds can be substituted. The initial chemical reaction is shown as equation 1 below specifically for preparing the gallium trichloride compound: 7 I

Ga Halogen Source GaCl ('1) The other'r'ne'tals (impurities) in the gallium sludge likewise react with the halogenating agent to form the corresponding metal halide.

In the next process step purified dry liquid ammonia is used to dissolve and react with the gallium trichloride to form what believed to be a complex of ammonia and gallium chloride, referred to herein as gallium chloride/ammoniate. The precise chemistry of this reaction is not understood; however, it is believed that the complex is formed as shown in equation 2 below.

GaCl +NH Ga(Cl),(NI-I 2,

wherein x and y are numbers which represent the molecular proportions of halide and ammonium ion, respectively. The gallium halide ammonium complex is soluble in the liquid ammonia. However, some of the other halides of the metal impurities in the gallium sludge are neither reactive nor soluble in the liquid ammonia. For example, lead, calcium and zinc chloride are practically insoluble in liquid ammonia. Boron trichloride, however, undergoes complete ammonalysis to boron triarnide or diboron triimide. Aluminum chloride reacts with liquid ammonia to a basic salt of indefinite composition.

The solid impurities of the halides of metals such as lead, zinc and calcium and other impurities mentioned hereinbefore do not readily undergo reaction with liquid ammonia and remain as solid precipitates in the ammonia solution of gallium halide ammoniate. Filtration of this solution physically removes a large proportion of the solid impurities. The ammoniacal solution is then evaporated to dryness and under relatively mild conditions, and under such conditions which will permit recovery of the ammonia solvent. The gallium halide ammonium complex remains as a solid aftefevaporation. The ammonia may be repurified prior to redissolving the solids formed by evaporation. Upon subsequent dissolution steps proportions of the metallic halide ammonium complexes or compounds remain insoluble and may be filtered out of the solution. Subsequent dissolution and filtration steps remove substantially all of the impurities originally present in the gallium sludge. In the preferred embodiment of this invention, the solution filtration and evaporation steps are repeated until no further solids are removed by filtration.

After the solid metal halide ammonium complexes or compounds have been substantially removed by the above described dissolution and filtration procedures, the gallium chloride ammonium complex is again dissolved in purified liquid ammonia. The gallium complex solution is then electrolyzed. The electrodes may be any conventional material. However, standard graphite or platinum electrodes are preferred for the anode and gallium or platinum, stainless steel, zirconium, tungsten, tantalum electrodes are preferred for the cathode. The cathode current density is preferably in the range of between about 0.0l and 0.1 amperes per square centimeter. Gallium metal is reduced in the electrolytic reaction and is precipitated or plated on the cathode in extremely pure form. The electrolytic reactions are shown below in equation 3.

It is further noted that metallic gallium as well as Ga and Ga(Nl-l are inert to and do not react with ammonia. Thus, gallium is readily electrodeposited on the cathode in a high state of purity from the gallium halide ammonium complex solution in liquid ammonia.

It is also noted that the equipment useful in the preparation of the hyperpure gallium should be inert to the liquid ammonia and other materials used in this process. For example, containers should be made of teflon, quartz and other materials known to be inert as described. It is preferred that during the course of the purification process, and especially during the electrolytic reaction, air or oxygen is excluded in order to prevent oxidation of the gallium metal. An inert atmosphere such as nitrogen is useful. Temperature 40C 80C, Presssure: atmospheric.

The preferred concentrations for the gallium halide ammonium complex in ammonia is in the range of 0.05 to 0.5 molar.

. The following is a specific example of the method of practicing this invention.

About 1000. grams of gallium arsenide scrap material containing gallium arsenide and oxides of gallium and arsenic is dried and it is chlorinated with chlorine gas at 400C. To convert the oxides into chlorides, the temperature of chlorination is subsequently increased to 800C.

The reaction products obtained from the chlorinator are distilled in a stream of argon. About 400 ml of crude gallium chloride are recovered. About 100 grams of crude GaCl are mixed with 1 liter of liquid ammonia at C. The solution is filtered. The filtrate is evaporated to dryness and redissolved in 1 liter of liquid ammonia and refiltered. This step is repeated three times and finally the filtrate is electrolyzed using platinum electrodes at current density of about 0.05 amperes per square centimeter of the cathode. About 30 grams of gallium are deposited on the cathode. The cathode is removed from the electrolytic cell and allowed to warm in an argon atmosphere and gallium is stripped off.

In addition to gallium, the metals germanium and indium can likewise be purified to the hyperpure form.

It will be understood that it is intended to cover all changes and modifications of the example of the invention herein chosen for the purpose of illustration which do not constitute departures from the spirit and scope of the invention.

What is claimed is:

l. A method for preparing hyperpure metal selected from the group consisting of gallium, germanium and indium comprising the steps of:

' a. halogenating one of said metals which contains less than about 2% impurities;

b. dissolving said halogenated metal in liquid ammoc. filtering undissolved solids from said solution;

d. electrolyzing said resolution of said solid material;

and

e. recovering hyperpure metal from the cathode of the electrolytic cell.

2. The process of claim 1 wherein steps comprising the following steps are performed on said filtered solution prior to said electrolyzing step:

a. evaporating said solution to dryness to provide solid material; and

b. redissolving said solid material in liquid ammonia.

3. The process of claim 2 wherein said metal is gallium. 

1. A METHOD FOR PREPARING HYPERPURE METAL SELECTED FROM THE GROUP CONSISTING OF GALLIUM, GERMANIUM AND INDIUM COMPRISING THE STEPS OF: A. HALOGENATING ONE OF SAID METALS WHICH CONTAINS LESS THAN ABOUT 2% IMPURITIES, B. DISSOLVING SAID HALOGENATED METAL IN LIQUID AMMONIA, C. FILTERING UNDISSOLVED SOLIDS FROM SAID SOLUTION, D. ELECTROLYZING SAID RESOLUTION OF SAID SOLID MATERIAL, AND E. RECOVERING HYPERPURE METAL FROM THE CATHODE OF THE ELECTROLYTIC CELL.
 2. The process of claim 1 wherein steps comprising the following steps are performed on said filtered solution prior to said electrolyzing step: a. evaporating said solution to dryness to provide solid material; and b. redissolving said solid material in liquid ammonia.
 3. The process of claim 2 wherein said metal is gallium. 